Any aerosol in concentrated quantities is harmful. In short, if you've constructed a spray booth without ventilation, you will be concentrating the aerosolized particles in a smaller space. It would be better to eliminate the booth entirely in that case. In my military career, I worked as a test officer in an assignment where toxic agents were quantified by both toxicity and concentration. The science is simple-- the higher the parts per million (PPM) or parts per billion (PPB) of milli-microns of agent (in this case hobby paint) to the smaller the cubic foot (CF) of air, the more toxic the mixture. Given this principle, you could spray an ounce of enamel or laquer based paint in a 3000 CF ventilated room, and have almost no hazard. On the other hand, you could spray a 1/2 ounce of acrylic paint in a 4 CF box and have a much higher hazard level. Therefore, if you are hoping to contain the paint inside a spray booth of 4 CF, without ventilation, you may have a problem as you'll be breathing the same air for 2-3 hours a week--unless you leave that window open. I suspect the small fume extractors you refer to won't have enough air volume passing through them to extract harmful particles (PPM/PPB) over a 2-3 hour painting session. One way to check is to find out what the Cubic Feet per Minute (CFM) of airflow through the filter system is, and compare it to the CF of your spray booth. Your fan and filter CFM should at least be equal to the CF size of the booth as a rule of thumb. Having given this lengthy explanation, I simply paint enamels and lacquers in my garage, which is about 3000 CF, and I leave the garage door open about 3 ft with a box fan running while I paint. If your room is large enough, and you can open a window, you'll probably be OK. Just remember it's not the toxicity of the paint that counts, it's the concentration-- a spray booth without a ventilation system is a concentrator. There is another solution-- and that would be to obtain an MSA approved mask, but that won't vacate the air in the room, it will only prevent inhalation of the paint particles.